Safety switches with a read head and an actuator, each having a first and/or second component set with electric and/or electronic structural elements which may be caused to interact without electric contact and as a result control the safety switch, are disclosed in DE 197 11 588 A1, for example. Such safety switches may be used to monitor movable protective devices, e.g., doors, covers, grates, and the like of machinery and equipment. As a rule, the safety switch interrupts one or more electric circuits when the relevant protective device is transferred to a safer operating state, is switched off, for example, or switching on of the device is prevented.
The actuator generally is introduced in a channel formed by the read head. When assembled, the actuator in the read head may be mechanically locked and as a result the protective device may be kept locked. Locking in conventional systems is accomplished as a result of extension of a rod directly through an opening in the actuator or blocking of a control gear in the read head operating in conjunction with the actuator. In order for it to be possible to apply the locking forces required, 1000 N, for example, as a function of the application the locking system, and accordingly, the safety switch, must be designed to be sufficiently rugged from the mechanical viewpoint.
As an alternative or in addition to the mechanical locking, the locking may be effected by electromagnetic forces, which are, of course, heavily dependent on the distance between the electromagnet and the associated counterelement. In particular, the electromagnetic forces decrease sharply with increase in distance. The fouling of the safety switch and of the locking system which occurs may impair provision of high locking forces and other aspects of operation in fouling environments, such as in the vicinity of metal-cutting machine tools.
DE 199 53 898 A1 discloses an access protection device having a U-shaped holding magnet. A configuration having a rotatable U-shaped permanent magnet is described. A first reed contact detects the rotary position of the permanent magnet, that is, engagement or disengagement of the magnetic locking action. A second reed contact detects the presence or absence of the counterelement, that is, whether the protective door is closed or not.
DE 198 40 620 C1 discloses a contact-free safety switch having a Hall sensor mounted on the read head and a permanent magnet mounted on the actuator. The Hall sensor monitors the closed position. The switching threshold of the Hall sensor is adjustable by a trimming resistor.